It is important that a proper operating temperature be maintained within an enclosure that houses an electrical assembly or communications equipment. The equipment often needs to be maintained at or above a minimum operating temperature. For example, certain electronic devices will not operate below 0.degree. C. Yet due to the intended use for the equipment (e.g., a cell site base station for a cellular telephone system), the equipment may have to be mounted outdoors where the ambient temperature may at times drop to below 0.degree. C. or other minimum operating temperature for the equipment.
Traditionally, for equipment mounted outdoors, heaters have been placed in the housings for keeping the equipment sufficiently warm. However, traditional heaters have presented considerable difficulties. One difficulty relates to disabling of the equipment in a cold environment which may be necessitated, for example, when the equipment is inspected for maintenance, repaired, or when there is a power outage. In these cases, it may be necessary to heat the equipment to the minimum operating temperature before reactivation, but to do so, the heater may have to be manually operated. Another difficulty arises because the equipment itself may have heat generating components, or it may be heated from sun rays hitting the assemblies in the outdoors. Traditional heaters are known to create fire hazards. Solar shields have been placed over the devices to prevent an increase in temperature from the sun, but these shields may adversely inhibit the dissipation of heat from the system. Thermostat controls have been placed on the enclosures and regulated externally, that is, in an effort to control the temperature within the enclosure, but regulating the temperature in this fashion produces unstable performance, is not reliable, and generates a relatively high failure rate. Another difficulty with traditional heaters is that presently available heater pads are comprised of materials that will not operate at narrow voltage ranges, placing limits on use of the devices. For example, these systems are not operable for cell site base stations located in Europe or other parts of the world where a high operating voltage (e.g., &gt;110V), is required.
The challenges presented by the outdoor environment coupled with the increased use of outdoor-mounted communications equipment has led to innovative designs for the electrical housing units themselves. For example, a new housing design having heat dissipation fins is described in U.S. Pat. application Ser. No. 09/008,726, "Heat Dissipation Structure for an Electrical Assembly," filed Jan. 1, 1998 by Aakalu et al, and assigned to the present assignee, which is hereby incorporated by reference. See also U.S. Pat. application Ser. No. 08/976,708, "Multi-Layered Polymer Structure For Fabricating Housings For Communications And Electrical Products," by Zimmerman et al., and U.S. Pat. application Ser. No. 08/976,713, "Dual Network Housing Device And Improved Method For Enclosing Networks" both filed Nov. 24, 1997, and assigned to Lucent Technologies, and U.S. Pat. No. 3,900,700 to Gaudet, "Protective Enclosure." All three of the patents cited immediately above are incorporated herein by reference. These devices provide improved heat dissipation features and flame retardant materials. However, it would be advantageous to provide an improved housing that avoids use of traditional heater elements and their associated drawbacks.